always the end of the month. dealer are more eager to sell the car cheaper in order to make the sales figure look good or meet some quota.
second best is end of the week, same reason.
there are a lot of redesigned model for different make and model, so buy the 2001 model, not the newly designed model for better price.
For example: Toyota Camry and Nissan Altima are two that have a great 2002 model, they do have a lot of 2001 inventories left.